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A Brief History of Time Measurement

Stage: 2, 3, 4 and 5

Article by Leo Rogers

Ever since man first noticed the regular movement of the Sun and
the stars, we have wondered about the passage of time. Prehistoric
people first recorded the phases of the Moon some 30,000 years ago,
and recording time has been a way by which humanity has observed
the heavens and represented the progress of civilization.

Natural Events

The earliest natural events to be recognised were in the heavens,
but during the course of the year there were many other events that
indicated significant changes in the environment. Seasonal winds
and rains, the flooding of rivers, the flowering of trees and
plants, and the breeding cycles or migration of animals and birds,
all led to natural divisions of the year, and further observation
and local customs led to the recognition of the seasons.

Measuring time by the Sun, the Moon and the Stars

As the sun moves across the sky, shadows change in direction
and length, so a simple sundial can measure the length of a day. It
was quickly noticed that the length of the day varies at different
times of the year. The reasons for this difference were not
discovered until after astronomers accepted the fact that the earth
travels round the sun in an elliptic orbit, and that the earth's
axis is tilted at about 26 degrees. This variation from a circular
orbit leads to the Equation of Time (see 'Note 2' below) which
allows us to work out the difference between 'clock' time and
'sundial time'.

Another discovery was that sundials had to be specially made
for different latitudes because the Sun's altitude in the sky
decreases at higher latitudes, producing longer shadows than at
lower latitudes. Today, artists and astronomers find many ways of
creating modern sundials.

A sundial with roman numerals. As you look at this dial, which
direction are you facing?

The progress of the sun can be recorded using the four faces of
this cube. Can you discover the orientation of these faces?

Wall Sundial

Prehistoric carving said to represent the Orion
constellation

The oldest image of a star pattern, the constellation of Orion,
has been recognised on a piece of mammoth tusk some 32,500 years
old. The constellation Orion is symbolized by a man standing with
his right arm raised and a sword at his belt and can be seen
throughout the world at different times of the year. Orion was the
sun god of the Egyptians and Phonecians and called the 'strong one'
by the Arabs. In parts of Africa, his belt and sword are known as
'three dogs chasing three pigs' and the Borana people of East
Africa based a sophisticated calendar on observations of star
clusters near Orion's belt. Orion contains some of the brightest
stars in the southern part of the winter sky in the northern
hemisphere and can be seen later in the southern hemisphere.

The three stars of Orion's belt and
the red star of his right arm can be easily recognised

The earliest Egyptian Star Map is about 3,500 years old and shows
the most unusual conjunction of the planets (Venus, Mercury, Saturn
and Jupiter) in the constellation of Orion and the occurrence of a
solar eclipse that happened in 1534 BCE.

Babylonian records of observations of heavenly events date
back to 1,600 BCE. The reason for adopting their arithmetic system
is probably because 60 has many divisors, and their decision to
adopt 360 days as the length of the year and 3600 in a circle was
based on their existing mathematics and the convenience that the
sun moves through the sky relative to fixed stars at about 1degree
each day.

The constellation Taurus, the bull, a symbol of strength and
fertility, figures prominently in the mythology of nearly all early
civilizations, from Babylon and India to northern Europe. The
Assyrian winged man-headed bull had the strength of a bull, the
swiftness of a bird and human intelligence.

From about 700 BCE the Babylonians began to develop a mathematical
theory of astronomy, but the equally divided 12-constellation
zodiac appears later about 500 BCE to correspond to their year of
12 months of 30 days each. Their base 60 fraction system which we
still use today (degrees / hours, minutes and seconds) was much
easier to calculate with than the fractions used in Egypt or
Greece, and remained the main calculation tool for astronomers
until after the 16th century, when decimal notation began to take
over.

The earliest archaeological evidence of Chinese calendars appears
about 2,000 BCE. They show a 12 month year with the occasional
occurrence of a 13th month. However, traditional Chinese records
suggest the origin of a calendar of 366 days depending on the
movements of the Sun and the Moon as early as 3,000 BCE. Over such
a long period of observation, Chinese astronomers became aware that
their calendar was not accurate, and by the second century CE it
was recognised that the calendar became unreliable every 300 years.
This problem is called Precession and was recorded by
Chinese historians in the fourth and fifth centuries CE. In the
fifth century CE the scholar Zu Chongzi created the first calendar
which took precession into account, and the most comprehensive
calendar was the Dayan Calendar compiled in the Tang Dynasty
(616-907 CE) well ahead of any such development in Europe.

Precession is due to
the gradual movement of the Earth's rotational axis in a circle
with respect to the fixed stars. This movement produces a slow
'wobble' which means that the positions of the stars complete a
cycle of about 26,000 years.

The Earth's axis completes a circuit about once every 26,000
years

In the Mediterranean, Hipparchus made the earliest calculations of
precession in about 160 BCE. The problem was taken up by
astronomers in the Middle East and India who recognized that
precession gradually altered the length of the year. Calendars have
had to be altered regularly. In 325 CE the spring (vernal) equinox
had moved to March 21. The Emperor Constantine established dates
for the Christian holidays, but Easter is based on the date of the
vernal equinox which varies every year because the equinox is an
astronomical event. By 1582 the vernal equinox had moved another
ten days and Pope Gregory established a new calendar, and this
change is the reason for having an extra day in every leap year.
However, there are still small changes accumulating, and one day we
shall have to adopt a new calendar!

Inventions for measuring and regulating time

The early inventions were made to divide the day or the night into
different periods in order to regulate work or ritual, so the
lengths of the time periods varied greatly from place to place and
from one culture to another.

Oil Lamps

There is archaeological evidence of oil lamps about 4,000 BCE,
and the Chinese were using oil for heating and lighting by 2,000
BCE. Oil lamps are still significant in religious practices,
symbolic of the journey from darkness and ignorance to light and
knowledge. The shape of the lamp gradually evolved into the typical
pottery style shown. It was possible to devise a way of measuring
the level in the oil reservoir to measure the passing of
time.

Candle Clocks

Marked candles were used for telling the time in China from
the sixth century CE. There is a popular story that King Alfred the
Great invented the candle clock, but we know they were in use in
England from the tenth century CE. However, the rate of burning is
subject to draughts, and the variable quality of the wax. Like oil
lamps, candles were used to mark the passage of time from one event
to another, rather than tell the time of day.

Water Clocks

The water clock, or clepsydra, appears to have been invented
about 1,500 BCE and was a device which relied on the steady flow of
water from or into a container. Measurements could be marked on the
container or on a receptacle for the water. In comparison with the
candle or the oil lamp, the clepsydra was more reliable, but the
water flow still depended on the variation of pressure from the
head of water in the container.

Astronomical and astrological clock making was developed in
China from 200 to 1300 CE. Early Chinese clepsydras drove various
mechanisms illustrating astronomical phenomena. The astronomer Su
Sung and his associates built an elaborate clepsydra in 1088 CE.
This device incorporated a water-driven bucket system originally
invented about 725 CE. Among the displays were a bronze
power-driven rotating celestial globe, and manikins that rang
gongs, and indicated special times of the day.

Improvements were made to regulate
the flow by maintaining a constant head of water

Su Sung's astronomical water
clock

Sandglass

Hour Glasses or Sandglasses

As the technology of glass-blowing developed, from some time in the
14th century it became possible to make sandglasses. Originally,
sandglasses were used as a measure for periods of time like the
lamps or candles, but as clocks became more accurate they were used
to calibrate sandglasses to measure specific periods of time, and
to determine the duration of sermons, university lectures, and even
periods of torture.

The Division of the Day and the Length of the 'Hour'

An Egyptian sundial from about 1,500 BCE is the earliest
evidence of the division of the day into equal parts, but the
sundial was no use at night. The passage of time was extremely
important for astronomers and priests who were responsible for
determining the exact hour for the daily rituals and for the
important religious festivals, so a water clock was invented.

The Merkhet

The Egyptians improved upon the sundial with a 'merkhet', one
of the oldest known astronomical instruments. It was developed
around 600 BCE and uses a string with a weight as a plumb line to
obtain a true vertical line, as in the picture. The other object is
the rib of a palm leaf, stripped of its fronds and split at one
end, making a thin slit for a sight.

A pair of merkhets were used to establish a North-South
direction by lining them up one behind the other with the Pole
Star. Viewing the plumb lines through the sight made sure the two
merkhets and the sight were in the same straight line with the Pole
Star. This allowed for the measurement of night-time events with a
water clock when certain stars crossed the vertical plumb line (a
'transit line'), and these events could then be recorded by
'night-time lines' drawn on a sundial.

See also 'Note 1' below.

An Egyptian Merkhet. The wooden
upright has a notch to use as a sight when using two plumb
lines

There are various theories about how the 24 hour day developed. The
fact that the day was divided into 12 hours might be because 12 is
a factor of 60, and both the Babylonian and Egyptian civilisations
recognised a zodiac cycle of 12 constellations. On the other hand,
(excuse the pun) finger-counting with base 12 was a possibility.
The fingers each have 3 joints, and so counting on the joints gives
one 'full hand' of 12.

In classical Greek and Roman times they used twelve hours from
sunrise to sunset; but since summer days and winter nights are
longer than winter days and summer nights, the lengths of the hours
varied throughout the year.

In about 50 BCE Andronikos of Kyrrhestes, built the Tower of Winds
in Athens. This was a water clock combined with Sundials positioned
in the eight principal wind directions. By then it was the most
accurate device built for keeping time.

The Tower of the Winds in Athens
contained a clepsydra and shows the North-East, North and
North-West deities in this picture

Hours did not have a fixed length until the Greeks decided they
needed such a system for theoretical calculations. Hipparchus
proposed dividing the day equally into 24 hours which came to be
known as equinoctial hours. They are based on 12 hours of daylight
and 12 hours of darkness on the days of the Equinoxes. However,
ordinary people continued to use seasonally varying hours for a
long time. Only with the advent of mechanical clocks in Europe in
the 14th Century, did the system we use today become commonly
accepted.

Earliest mechanical clock

Mechanical clocks replaced the old water clocks, and the first
clock escapement mechanism appears to have been invented in 1275.
The first drawing of an escapement was given by Jacopo di Dondi in
1364. In the early-to-mid-14th century, large mechanical clocks
began to appear in the towers of several cities. There is no
evidence or record of the working models of these public clocks
that were weight-driven. All had the same basic problem: the period
of oscillation of the mechanism depended heavily on the driving
force of the weights and the friction in the drive.

In later Mediaeval times elaborate clocks were built in public
places. This is the Astronomical clock in Prague, parts of which
date from about 1410.

This mechanism illustrates a basic
escapement. The weight rotates the drum which drives the toothed
wheel which gives the mechnism its "tick-tock" movement

Prague Astronomical Clock

Showing the Zodiac Circles and
early versions of the digits 2, 3, 4 and 7

The earliest surviving spring driven clock can be found in the
science museum in London and dates from about 1450. Replacing the
heavy drive weights with a spring permitted smaller and portable
clocks and watches.

More Accurate Mechanical Clocks

Christiaan Huygens made the first pendulum clock, regulated by
a mechanism with a "natural" period of oscillation in 1656. Galileo
studied pendulum motion as early as 1582, but his design for a
clock was not built before his death. Huygens' pendulum clock had
an error of less than 1 minute a day, and his later refinements
reduced his clock's errors to less than 10 seconds a day.

There was no device for keeping accurate time at sea until
John Harrison, a carpenter and instrument maker, refined techniques
for temperature compensation and found new ways of reducing
friction. By 1761, he had built a marine chronometer with a spring
and balance wheel escapement that kept very accurate time. With the
final version of his chronometer, which looked like a large pocket
watch, he achieved a means of determining longitude to within
one-half a degree.

It was not until 1884 that a conference at Greenwich reached
agreement on global time measurement and adopted Greenwich Mean
Time as the international standard. Today we rely on atomic clocks
for our most accurate time measurements.

The pendulum moves the lever which
creates the rocking movement of the escapement

N.B.
Pedagogical notes related to measurement and time can be found by
clicking on the "Notes" tab at the top of
this article.

Supporting notes

Note 1

When you think about the problem - we can find due South
easily from the sun at midday. Looking at the night sky, we
eventually deduce that there is a fixed point in the heavens around
which all the stars rotate once every day (24 hours). This is where
we find the 'Pole Star' (from the Great Bear or Ursa Major, measure
the distance of about four lengths of the two stars at the end,
'the pointers' to find Polaris). This is the Celestial Pole - which
was different for the Egyptians from today because of the
phenomenon of Precession.

Up to about 1,900 BCE the Celestial Pole was Thuban a star in
the 'tail' of the constellation Draco. By 1,000 BCE it was Thuban
in the constellation Ursa Minor. Today Polaris is the last star in
the 'tail' of Ursa Minor

Note 2

'Sun time' and 'clock time' are different. Sun time is based on
the fact that the sun reaches its highest point (the meridian), in
the middle of the day, and on the next day at its highest point, it
will have completed a full cycle. However, the time between the sun
reaching successive meridians is often different from clock time.
According to clock time, from May to August, the day is close to 24
hours, but in late October the days are about 15 minutes shorter,
while in mid February the days are about 14 minutes longer. For our
daily routines, it is important to have a constant 'clock time' of
24 hours. This variation is called the 'Equation of Time' and shows
the relationship between sun time and clock time. The variation has
two causes; the plane of the Earth's equator is inclined to the
Earth's orbit around the Sun, and the orbit of the Earth around the
sun is an ellipse and not a circle. The National Maritime Museum
website shows two separate graphs for these causes, and a third
graph where they are combined to give the full correction.

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